The present invention relates to a novel process for the preparation of bile acid derivatives in which an amino group is present at the 3xcex2 position.
The most important bile acids are represented in the following scheme 1: 
More particularly, the invention relates to a process for the preparation of compounds of formula (I) from compounds of formula (II): 
in which
R1 is H or OH;
R2 is H, xcex1-OH, or xcex2-OH; and
R3 is a straight or branched C1-C4 alkyl group, or a benzyl group.
The compounds of general formula (I) are important intermediates for the preparation of compounds useful for a number of pharmaceutical applications. They are, for example, used for the preparation of inhibitors of bile acids intestinal absorption (see EP-A-0489423) or as carriers for active compounds in the enter-hepatic circulation (see EP-A-0417725).
Last, but not least, is the use thereof for the preparation of contrast agents for medical diagnosis using Magnetic Resonance, such as those described in WO-A-95/32741, resulting from the conjugation of a bile acid with a chelating agent, which are capable of chelating the ions of paramagnetic bi- and trivalent metals, in particular the gadolinium ion, or in the publication: Anelli P. L. et al., Acta Radiologica, 38, 125, 1997.
EP-A-614,908 discloses the preparation of the derivatives of general formula (I), comprising the following steps:
a) formation of phthalimido derivatives of general formula (III) by reacting compounds of general formula (II) with phthalimide; 
b) treatment of compounds of general formula (III) with hydrazine hydrate or phenylhydrazine;
c) subsequent treatment of the reaction products from step
b) with mineral acids to form the addition salts, and
d) liberation from the salt to give the compounds of general formula (I).
As already widely discussed in EP-A-614,908, the formation of the phthalimido derivative is carried out by means of the well known Mitsunobu reaction (see Synthesis, 1, 1981; Org. React. Vol. 42, 335 (1992)), namely in the presence of a suitable phosphine and DEAD (diethylazodicarboxylate) or DIAD (diisopropylazodicarboxylate), in an organic solvent such as dioxane or tetrahydrofuran, at a temperature which ranges from 20 to 50xc2x0 C. The Mitsunobu reaction yields the final products with inversion of configuration.
The process illustrated makes use of hydrazine hydrate or phenylhydrazine, particularly dangerous products due to their ascertained cancerogenicity.
It has now surprisingly been found that the reduction of the phthalimido group in the compounds of formula (III) can be advantageously carried out with sodium borohydride.
It is therefore the object of the present invention a process for the preparation of the compounds of general formula (I) comprising the reduction reaction of the compounds of general formula (III) in the presence of sodium borohydride, according to the following Scheme 2: 
in which
R1is H or OH;
R2 is H, xcex1-OH, or xcex2-OH; and R3 is a straight or branched C1-C4 alkyl group, or a benzyl group.
A further object of the present invention is the process for the preparation of the compounds of general formula (I) comprising the reduction of the compounds of general formula (III) in the presence of sodium borohydride, through formation of the novel compounds of general formula (IV), and subsequent deprotection by treating said compounds with acids, according to the following Scheme 3: 
in which R1, R2 and R3 have the meanings defined above.
Compounds of general formula (IV) are novel, useful intermediates in the process according to Scheme 3 and their recovery will be described in the Experimental Section.
Particularly preferred is the process for the preparation of compounds of formula (Ia), according to Scheme 3, starting from compounds of formula (IIIa), which are derivatives of cholic or deoxycholic acid, 
in which
R3 has the meanings defined above and
R4 can be a hydrogen atom or a hydroxy group.
A further object of the present invention is the process for the preparation of compounds of formula (Ib) according to Scheme 4, starting from compounds of formula (IIIb), which are deoxycholic acid derivatives, 
in which R3 has the meanings defined above.
Particularly preferred is the process for the preparation of compound (Ic), according to Scheme 5, starting from compound (IIIc), a deoxycholic acid derivative, 
The compounds of formula (IIIc) and (IVc), (3xcex2,5xcex2,12xcex1)-3-[1,3-dihydro-1,3-dioxo-2H-isoindol-2-yl)]-12-hydroxycholan-24-oic acid methyl ester and (3xcex2,5xcex2,12xcex1)-12-hydroxy-3-[[(2-hydroxymethyl)benzoyl]amino]cholan-24-oic acid methyl ester, respectively, are novel and they are useful intermediates for the preparation of compound of formula (Ic), a deoxycholic acid derivative.
The compounds of formula (III) are synthesized according to the general procedure already described above and exemplified in EP-A-614,908.
Preferred reaction conditions comprise:
1) selection of a reaction temperature from 15 to 25xc2x0 C., thus decreasing the amount of diisopropylazodicarboxylate to a small excess to the stoichiometric;
2) crystallization of phthalimido derivatives (III) from MeOH instead of 2-PrOH, thereby reducing the volume of the crystallization solvent by at least four times.
The reduction of phthalimido derivatives with NaBH4 has never before been described in the field of the compounds of the present invention, but references exist in the literature, (T. W. Greene; P. G. M. Wuts xe2x80x9cProtective groups in Organic Synthesisxe2x80x9d; 3rd Ed John Wiley and sons, New York, 1999), reporting the use of a large excess of this reducing agent (5-10 mol) to deprotect a phthalimido group.
The teaching contained in the most important paper [Osby, Tetrahedron Letters, Vol. 25, 2093 (1984)] i.e. the use of 2-PrOH/H2O=6/1 as a reduction solvent proved to be definetly unsuitable for the process of the present invention. In fact, due to the poor solubility of compounds of formula (III), it was necessary to operate at temperatures around 40xc2x0 C. and with a 2.5% maximum concentration which is of course industrially unacceptable.
On the other hand, Osby already evidenced that the proposed conditions were not particularly suitable to deprotect phthalimido groups from derivatives containing an ester function. Osby himself in fact observed that the deprotection of this group in said derivatives was accompanied by reduction and hydrolysis of the ester group with consequent decrease in reaction yields.
The Applicant found that when reducing the compounds of formula (III) under the Osby conditions, compounds of formula (I) were recovered in around 65% yields, but contaminated by remarkable amounts of by-products, mainly the product from the reduction of the ester group at 24- to alcohol. This phenomenon is still significant even when markedly reducing the NaBH4 excess. In order to better understand the genesis of the by-products, the reaction carried out under the osby conditions was worked up at the end of the reduction with NaBH4 and before the acid hydrolysis and the side-products recovered by silica chromatography had the formulae (V)-(VIII). These by-products are of course the precursors of the impurities evidenced in compounds of formula (I) and account for the low reaction yields. 
In the Experimental section the recovery and the characterization of the various intermediates, in particular in the case of deoxycholic acid, will be reported.
Compounds of formula (IV) are substantially inseparable from compounds (V) by crystallization from different solvents. Not even the conversion of compounds (IV) into the corresponding acids of formula (VI) allows the elimination of the by-products of formula (V).
It has surprisingly been found that using dipolar aprotic solvents such as DMA (dimethylacetamide), DMF (dimethylformamide), DMSO (dimethylsulfoxide), NMP (N-methyl-pyrrolidone), in place of an alcoholic solvent, such as isopropanol described by Osby, these problems could be overcome. Particularly preferred are dimethylacetamide and N-methylpyrrolidone.
Using said solvents the concentration of compounds (III) could be increased up to more than 10%, without increasing the amount of side-products.
Moreover, under the conditions of the present invention, the NaBH4 excess could be decreased, compared with what reported above.
In particular, the molar equivalents of reducing agent can be lowered from a strong excess (about 5) to substantially stoichiometric values (1.2-0.85) to the substrate, without adversely affecting the reaction yields.
A further aspect of the present invention is the use, simultaneously with the dipolar aprotic. solvents, of a buffer solution at pH 7.5-9 which allows to control pH during the reaction. This allows to inhibit the hydrolysis of sensitive functions minimizing the formation of by-products (VI)-(VIII).
The compounds of formula (IV) are then transformed into those of formula (I) by treatment in dipolar aprotic or alcoholic solvent, optionally in mixture with water, and in the presence of a mineral or organic acid (such as acetic acid).
Particularly preferred is the use of HCl in methanol solution. The subsequent liberation of the free base from the salt is carried out by treatment of the solution in one of the above cited solvents with aqueous bases (such as NaOH, Na2CO3 . . . ).
As already cited above, compounds of formula (I) are useful for the preparation of medicaments for decreasing cholesterol plasmatic levels, (EP-A-417,725 or EP-A-489,423) or of contrast agents for the nuclear magnetic resonance diagnosis, as described in WO-A-95/32741.
For the latter, the known synthesis comprised the transformation of compounds (II) into the corresponding compounds (I) by intermediate formation of the azide at the 3xcex2 position, according to Mitsunobu reaction, as represented in the following Scheme 6: 
This Synthetic scheme is unsuitable for scaling-up, being azides potentially dangerous and the key reagent (diphenylphosphoryl azide) extremely expensive.
The present invention relates to a process for the preparation of chelating agents of general formula (IX), capable of chelating paramagnetic bi-trivalent metal ions, selected the group consisting of Fe(2+), Fe(3+), Cu(2+), Cr(3+), Gd(3+), Eu(3+), Dy(3+), La(3+), Yb(3+) or Mn(2+), 
in which
R1, and R2 have the meanings defined above;
R5 is a hydrogen atom or a (C1-C5) alkyl group unsubstituted or substituted with a carboxylic group;
X is the residue of a polyaminocarboxylic ligand and of derivatives thereof, selected from the group consisting of: ethylenediaminotetraacetic acid (EDTA) , diethylenetriaminopentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A), [10-(2-hydroxypropyl)-4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A), 4-carboxy-5,8,11-tris(carboxymethyl)-1-phenyl-2-oxa-5,8,11-triazatridecan-13-oic acid (BOPTA);
L is a residue of formula 
m is an integer from 1 to 10, wherein for values above 1, the values of p and q can differ in the monomeric units;
q is 0 or 1;
p can range from 0 to 10, p and q not being at the same time zero,
said process comprising the following steps: 
a) formation of compounds (III) starting from compounds (II) by reaction with phthalimide according to Mitsunobul""s procedure, at temperatures ranging from 15 to 25xc2x0 C., in the presence of an azodicarboxylate selected from DEAD (diethylazodicarboxylate) or DIAD (diisopropylazodicarboxylate) in amounts ranging from 1.1 to 1.3 molar equivalents, in a solvent selected from the group consisting of THF, dioxane, toluene and DMF;
b) reduction of compounds (III) with NaBH4 to give compounds (IV);
c) acidic hydrolysis of compounds (IV) followed by neutralization to give compounds (I);
d) condensation of compounds (I) with the reactive residues of the polyaminocarboxylic ligands defined above.
Particularly preferred is the process for the preparation of compounds (IXa), which are cholic or deoxycholic acid derivatives, according to the following Scheme 7: 
n which R3, R4 and R5 have the meanings defined above.
Furthermore, particularly preferred is the process for the preparation of compounds (X) , in which in formula (IXa) the residue X is DTPA substituted on the chain at the central position, R6 can be a hydrogen atom or a carboxylic group and the L chain, R4 and R5 have the meanings defined above. 
Particularly preferred is the process for the preparation of compounds (Xa) 
in which the L chain, R4 and R5 have the meanings defined above.
Furthermore, particularly preferred is the process for the preparation of the following novel compound, of general formula (Xa):
[3xcex2(S) ,5xcex2,12xcex1,]-3-[[4-[bis[2-[bis(carboxymethyl)amino]ethyl]amino]-4-carboxy-1-oxobutyl]amino]-12-hydroxycholan-24-oic acid; 
Other compounds belonging to this class, whose preparation has already been described in WO-A-95/32741, are the following:
[3xcex2(S),5xcex2,7xcex1,12xcex1]-3-[[4-[bis[2-[bis(carboxymethyl)amino]ethyl]-amino]-4-carboxy-1-oxobutyl]amino]-7,12-dihydroxycholan-24-oic acid; 
[3xcex2(S),5xcex2,7xcex1,12xcex1]-3-[[4-[[5-[bis[2-[bis(carboxymethyl)amino]-ethyl]amino]-5-carboxypentyl]amino]-1,4-dioxobutyl]amino]-7,12-dihydroxycholan-24-oic acid. 
It is moreover preferred the process for the preparation of the following compounds of formula (Xb), in which in formula (X) R6 is a hydrogen atom, and R4, R5 and L have the meanings defined above. 
Also preferred is the process for the preparation of the following novel compound, belonging to the class of general formula (Xb):
(3xcex2,5xcex2,7xcex1,12xcex1)-3-[[[bis[2-[bis(carboxymethyl)amino]ethyl]amino]acetyl]amino]-7,12-dihydroxycholan-24-oic acid 
Other compounds belonging to this class, already described in WO-A-95/32741, are the following:
(3xcex2,5xcex2,7xcex1,12xcex1)-3-[[[[[bis[2-[bis(carboxymethyl)amino]ethyl]amino]acetyl]amino]acetyl]amino]-7,12-dihydroxycholan-24-oic acid; 
(3xcex2,5xcex2,7xcex1,12xcex1)-3-[[6-[[[bis[2-[bis(carboxymethyl)amino]ethyl]amino]acetyl]amino]-1-oxohexyl]amino]-7,12-dihydroxycholan-24-oic acid. 
Particularly preferred is also the process for the preparation of compounds of formula (XI), in which in formula (IXa) the residue X derives from DTPA, and R4, R5 and L have the meanings defined above. 
Other compounds belonging to this class, whose preparation was already described in WO-A-95/32741, are the following:
(3xcex2,5xcex2,7xcex1,12xcex1)-3-[[N-[N-[2-[[2-[bis(carboxymethyl)amino]ethyl]-(carboxymethyl)amino]ethyl]-N-(carboxymethyl)glycyl]glycyl]amino]-7,12-dihydroxycholan-24-oic acid; 
18-[[(3xcex2,5xcex2,7xcex1,12xcex1)-23-carboxy-7,12-dihydroxy-24-norcholan-3-yl]amino]-3,6,9-tris(carboxymethyl)-11,18-dioxo-3,6,9,12-tetraazaoctadecanoic acid. 
Particularly preferred is also the process for the preparation of the following compounds of formula (XII), in which in formula (IXa) the residue X is DOTA, and R4, R5 and L have the meanings defined above. 
Furthermore, preferred is the process for the preparation of compounds of formula (XIII), in which in formula (IXa) the residue X is EDTA, and R4, R5 and L have the meanings defined above. 
Particularly preferred is the process for the preparation of the following compounds of formula (XIII):
[3xcex2(S) ,5xcex2,7xcex1,12xcex1]-3-[[4-[[5-[[2-[bis(carboxymethyl)amino]ethyl]-(carboxymethyl)amino]-5-carboxypentyl]amino]-1,4-dioxobutyl]amino]-7,12-dihydoxycholan-24-oic acid 
[3xcex2(S),5xcex2,12xcex1]-3-[[4-[[2-[[bis(carboxymethyl)amino]ethyl](carboxymethyl)amino]-4-carboxy-1-oxobutyl]amino]-12-hydroxycholan-24-oic acid 
[3xcex2(S),5xcex2]-3-[[4-[[2-[[bis(carboxymethyl)amino]ethyl](carboxymethyl)amino]-4-carboxy-1-oxobutyl]amino]-12-oxocholan-24-oic acid 
The experimental conditions used will be illustrated in detail in the Experimental Section.